Cathode for electron discharge devices



Patented July 23, 1940 UNITE s'r'res CATHOD'E FOR ELECTRON DISCHARGEDEVICES Victor 0. Allen, Madison, N. J., assignor to Radio Corporationof America, a corporation of Delaware No Drawing. Application June 3,1939,

Serial No. 277,325

8 Claims.

My invention relates to electron discharge devices, more particularly toimprovements in thermionic electron emitting cathodes for such devices.

In conventional electron discharge devices with the filament type ofthermionic cathode commercial use has been made of pure tungstenfilaments, activated thoriated tungsten filaments, and oxide coatednickel filaments. The pure tungsten filal0 ment cathode is a very stableemitter and is efiicient but must be operated at very high temperaturesto obtain ample emission. The activated thoriated tungsten cathode, suchas described in United States patent to Langmuir 1,244,216, may

be operated at lower temperatures than a pure tungsten filament and ismore efficient, but is easily poisoned by oxygen and other gases whichadversely affect the emission, is not very stable when operated at highvoltages, and requires special processing and seasoning. It has alsobeen found that, when operated at ultra-high frequencies, for exampleabove 15 or 16 megacycles, emission becomes erratic and there issometimes a total loss of emission. In addition, the activated thori- 35ated filament is apt to be brittle, and if operated at too high atemperature quickly deactivates and loses its ability to emit asufiicient number of electrons to be useful. The oxide coated nickelcathode operates at a much lower temperature than either kind oftungsten cathode and is efficient, but cannot be satisfactorily operatedin tubes in which very high platevoltages, such as 1000 volts, are used,and requires considerable processing and seasoning.

Thoriated molybdenum has also been thoroughly investigated as a filamentcathode emitter, but has proved to be rather unstable, is quitesensitive to poisoning by gas within the tube, and must be operated atfairly high temperatures, be-

tween 1400 and 1600 C., to provide satisfactory emission. For these andother reasons thoriated molybdenum has not come into commercial use as acathode filament material.

In the conventional high vacuum or gas tube 4.3 operating at highvoltages, only filamentary type cathodes have heretofore been usedbecause the oxide coatings of the indirectly heated cathodes quicklydisintegrate at high voltages, and without these coatings no materialwas available which had sumcient emission unless raised to very hightemperatures which cannot be practically generated with the conventionalindirectly heated cathode construction.

To provide a ductile thermionic cathode which is a stable, efficient andlong lived emitter, at desirable operating temperatures, and which canbe satisfactorily used at high frequencies and high voltages as well asfor indirectly heated cathodes for high voltage gas or vacuum tubes, asdisclosed in United States Patent 2,144,250 issued to '5 me and JosephJohnson on January 17, 1939 and assigned to the Radio Corporation ofAmerica, I have successfully used thoriated molybdenum as a base, uponwhich chromium has been plated. While this is the preferred embodimentof my in- 10 vention, it has also been found that, in place ofmolybdenum, other equivalent refractory metals, such as tungsten ortantalum, having high melting points, preferably 2400" C. or higher, maybe used, as vwell as equivalents of thoria, such as 15 zirconium,uranium, cerium, titanium, vanadium, yttrium, or lanthanum.

However, in some cases under severe operating conditions when operatedat high frequencies there is a tendency for the chromium in the cathodesof the above described type to vaporize and condense on the cold partsof the tube, such as the walls, press and spacers, causing leakagebetween elements at high voltages. This also results in the reduction ofemission from the 25 cathode.

Hence it is the principal object of my invention to provide an improvedcathode of the type described in which the above noted undesirablefeatures are substantially reduced or eliminated. g

The thoriated molybdenum base may be prepared as a ductile metal inaccordance with United States Patent 1,082,933 to W. D. Coolidge, or bysquirting the finely divided material admixed with a-binder. Forexample, if molybde- 3 num is used I may add powdered nitrate of thoriato the powdered oxide of molybdenum before the reduction of the oxide ofmolybdenum, or I may add thoria to the oxide of molybdenum afterreduction but before consolidation of the metal powder by sintering andmechanical working to the solid state as described in the said Coolidgepatent. I have found in practice that a content of thoria of about 3% byweight of the molybdenum is satisfactory and that a filament of thisthoriated metal may be used although the percentage of the thoria may bevaried considerably. While thoria is preferred, equivalents of thoria,for example one of the group of metals consisting of zirconium, uranium,cerium, titanium, va- 50 nadium, yttrium and lanthanum may be used withmolybdenum to form a base and the base then chromium plated in aconventional chromium platingbath, the essential constituents of whichare'chromic acid and a sulphate. One ex- 5 ample of a satisfactory bathis 33 oz. chromic acid, containing at least 95% chromium oxide (CIOs)and not more than 0.2% S04, and .45 gram chemically pure sulphuric acid(H2804) in a gallon of water. The limits of thickness of the chromiumplating may be wide, although a very thin plating appears to be entirelysatisfactory. While the current density for plating the chromium on thethoriated metal base may vary between wide limits, for example, from afew hundredths of an ampere to over 1 ampere per square centimeter, forbest results a current density of about 1 ampere per square centimeterfor a period of one minute seems to produce the most satisfactoryresults. The resulting chromium plated thoriated wire may then beflashed in ammonia. The method used to flash the filament in ammoniacomprises positioning within a bell jar a cathode filament to betreated. Liquid ammonia was allowed to expand into the gas chamber toexpell the air within the jar. The filaments were then heated totemperatures of approximately 2000 K. for a period from thirty secondsto one minute. It is also desirable, though not necessary, to insurestability of emission, to season the filament by operating the electrondischarge device containing the filament with normal voltages appliedfor approximately fifteen minutes in an oscillating circuit.

While I do not wish to be limited to any particular theory, it isbelieved that the chromium applied electrolytically when heated with theammonia gas permitted the chromium to alloy with the molybdenum andtungsten more easily, the result being that the chromium could not be soeasily vaporized and deposited on the walls and other. parts of the tubeto cause the difficulties enumerated above.

A cathode made in accordance with my invention has a normal operatingtemperature several hundred degrees K. below the normal operatingtemperature of the conventional activated thoriated' tungsten cathode,and in the conventional type of tube operated at the customary platevoltage the same plate current can be obtained from our improved cathodeat its normal operating temperature with only about one-half the energyper unit area that must be used to obtain the same plate current fromthe conventional activated thoriated tungsten cathode at its normaloperating temperature.

When the cathodes are to be used in low'voltage tubes, for example under1000 volts, to increase the electron emissivity any of my cathodes abovedescribed may be coated with the usual electron emitting coating ofalkaline earth metal oxides, such as barium oxide and strontium oxide,which may be applied in a conventional manner by applying barium andstrontium carbonates and converting them. into the oxides.

A cathode made in accordance with my invention is a. very efficientemitter. It will operate at a comparatively low temperature (1200 C.)and very high voltages up to approximately 3000 volts can be used on theplate of a tube using my cathode without the cathode suffering any illeffects due to ion bombardment. These last two characteristics make myinvention particularly suitable for indirectly heated cathodes in highvoltage gas or vacuum tubes.

Cathodes made according to my invention are very satisfactory in tubesused for generating very high frequencies at high voltages and whileseasoning improves the initial stabiilty and oper ating characteristicsit is not essential in order to provide a cathode which is a goodemitter and operates satisfactorily. The resulting cathode is a verystable emitter and is not easily poisoned by gas. The cathode readilyrecovers its emitting characteristics even if accidentally operatedtemporarily at too high temperatures. Low voltage ionization will notdestroy .the tube due to ion bombardment, and the cathode will recoverits emission if temporarily impaired by being subjected to highvoltages.

While I do not wish to be limited to any particular theory it isbelieved that the chromium acts as a catalyst to facilitate the rapiddiiiusion of the electrom emitting metal, such as thorium, to thesurface of the filament for producing copious emission.

While I have indicated the preferred embodiment of my invention of whichI am now aware and have also indicated certain specific applications forwhich my invention may be employed, it will be apparent that myinvention isby no means limited to the exact forms or uses described,but that many variations. may be made in the particular structure used,the purpose for which it is employed, and the treatment given to it,without departing from the scope of my invention as set forth in theappended claims.

What I claim. as new is:

1. The method of treating a cathode body comprising a metal of highmelting point, thoria and electrolytically applied chromium, whichconsists of heating said body to a high temperature in ammonia gas.

2. The method of treating a cathode body comprising one of the group ofthe metals consisting of tungsten and molybdenum, and one of the groupof metals consisting of thorium, zirconium, uranium, cerium, titanium,vanadium, yttrium and lanthanum, and electrolytically applied chromium,which consists of heating the body to a high temperature in ammonia gas.

3. The method of treating a cathode body comprising one of the group ofmetals consisting of tungsten and molybdenum, and one of the group ofmetals consting of thorium, zirconium, uranium, cerium,titanium,vanadium, yttrium, and lanthanum, and electrolytically applied chromium,which consists of heating said body to substantially 2000 K. in ammoniagas.

4. The method of treating a cathode body comprising one of the group ofmetals consisting of tungsten and molybdenum, and one of the group ofmetals consisting of thorium, zirconium, uranium, cerium, titanium,vanadium, yttrium, and lanthanum, and electrolytically applied chromium,which consists of heating said body to substantially 2000 K. in ammoniagas for thirty seconds.

5. The method of treating a cathode body comprising one of the group ofmetals consisting of tungsten and molybdenum, and one of the group ofmetals consisting of thorium, zirconium, uranium, cerium, titanium,vanadium, yttrium, and lanthanum and electroyltically applied chromium,which consists of heating said body to substantially 2000 K. in ammoniagas for a period of time between thirty seconds and one minute.

6. The method of treating a cathode body comprising one of the group ofmetals consisting of tungsten and molybdenum, one of the group of metalsconsisting of thorium, zirconium, uranium, cerium, titanium, vanadium,yttrium, and lanthanum, and electrolytically applied chromium whichcomprises placing the body in a chamber,

admitting ammonia, gas to said chamber to expel air from said chamberand heating said body to a high temperature in the ammonia gas in saidchamber.

7. The method of treating a cathode body comprising one of the group ofmetals consisting of tungsten and molybdenum, one of the group of metalsconsisting of thorium, zirconium, uranium, cerium, titanium, vanadium,yttrium, and lanthanum, and electrolytically applied chromium whichcomprises placing the body in a chamber, admitting ammonia gas to saidchamber to expel air from said chamber, and heating said cathode body tosubstantially 2000 K. in

the ammonia gas for not less than thirty seconds.

8. The method of treating a cathode body comprising one of the group ofmetals consisting of tungsten and molybdenum, one of the group of metalsconsisting of thorium, zirconium, uranium, cerium, titanium, vanadium,yttrium, and lanthanum, and electrolytically applied chromium whichcomprises placing the body in a chamber, admitting ammonia gas to saidchamber to expel air from said chamber, and heating 10 said cathode bodyin the ammonia gas to substantially 2000 K. for a period of time betweenthirty seconds and one minute.

VICTOR O. ALLEN.

